Hi. I am having this same problem. I was able to solve it by changing the greeter theme of lightdm. To do this follow the steps below:
Open the lightdm configuration file with your favorite text editor, for example vim:
sudo vim /etc/lightdm/lightdm.conf
Change the line “greeter-session=lightdm-webkit2-greeter” to “greeter-session=lightdm-pantheon-greeter”. This lightdm theme has very good performance on my pc.